1. Field of the Invention
This invention .relates to apparatus and processes for reducing particulate contamination in plasmas.
2. Description of Related Art Problems in the Art and Related Art
Effectiveness of plasma etching in semiconductor device fabrication has been reduced by contamination problems.
Particulate contamination is a major problem encountered during plasma processing of microelectronic materials. By some accounts, 50% of current semiconductor chip yield loss may be attributed to direct or indirect effects of particulate contamination during fabrication. This fraction is expected to increase as device dimensions are reduced in future technologies. Particles that reduce process yields today range in size from the macroscopic to the sub-micron size.
Particulate contamination also has an extremely deleterious effect on the performance and reliability of microelectronic devices produced by plasma etching or deposition. Particulate contamination can result in device failure, poor film quality, changes in material resistivity, and impurity permeation. Further, as device dimensions are reduced, tighter control of the etching profile requires ever more stringent restrictions on the allowable particle contamination number, density, and size. To meet these requirements, tightly controlled, clean rooms are required to avoid particle deposition on product surfaces during wafer transport and handling.
Improvements in clean room technology and in the handling of in-process substrates (for semiconductors and other applications) have reduced the once appreciable introduction of particulates onto substrates during non-process exposure such as wafer handling and transfer. Particulate formation during process steps, including plasma processing, may now contribute a significant fraction of total contamination exposure with corresponding yield reduction. G.S. Selwyn, R. S. Bennett and J. Singh, "In-situ Laser Diagnostic Measurements of Plasma-Generated Particulate Contamination in RF Plasmas" J. Vac. Sci. Tech. A. Vol. 7 (4), pp2758-2765 (Jul/Aug 1989).
In addition, the industry trend is towards "integrated vacuum processing", or "multi-chamber processing". This means that surface contamination previously removed by wet or dry mechanical means will be more complex or impossible to remedy since it now requires removal of the substrate from the vacuum chamber. In multi-chamber tools, particulates which drop onto a wafer before, during, or at the completion of a process step may have an especially severe impact on subsequent process steps in that tool.
Recent studies in our laboratory have shown that certain etching plasmas can produce particulates which may be a significant source of product contamination and device failure. These experiments have shown that particles can be nucleated, grown, and suspended in a process plasma until they are significant in size. For example, particles are formed with sizes on the order of the submicron scale to hundreds of microns in diameter. The problem is that the particles ultimately fall onto devices being fabricated in the same manufacturing environment. If particles fall before or during film deposition or pattern transfer, then they can disrupt the process step. If they fall at the end of a process step, the particulates may disrupt subsequent process steps. These contaminants often produce defects which affect the device yield, performance and reliability. Similar results have been observed in deposition type plasmas (PECVD Silane), see R.M. Roth, K.G. Spears, G.D. Stein and G. Wong "Spatial Dependence of Particle Light Scattering in an RF Silane Discharge" App. Phys. Lett., 46 (3), 253-255 (1985)).
The effects of particulate contamination can be magnified when selective plasma etching processes are used. Certain plasma etching processes rely on a combination of feed gases and etching conditions to etch material surfaces on the wafer selectively. The chemical formation of particulates which are etched at a slow rate in these highly selective plasmas results in micromasking, or an irregular surface often referred to as "grass". This spike or hill of unetched material will also degrade the device performance and reduce process yield.
Contrary to common belief, presence of these particulates is not always due to material flaking from chamber walls, but may also be due to gas phase processes such as homogeneous nucleation. This suggests that particle contamination problems may not be eliminated solely by rigorous attention to clean room techniques and frequent cleaning of manufacturing equipment. Instead, since the plasma itself can result in product contamination, this problem may pose a "base level" of contamination even with the highest clean room technology. It is therefore important to develop means to operate the plasma while controlling or eliminating particle formation. Further, techniques are also necessary for removing particles, once present in a process.